Maven - why the built JAR can't find a jar that's in its lib folder?

Question

My pom.xml is:

<project xmlns="http://maven.apache.org/POM/4.0.0" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd">
<modelVersion>4.0.0</modelVersion>
 <groupId>b.c.m</groupId>
<artifactId>agent</artifactId>
<version>0.0.1-SNAPSHOT</version>

<properties>
    <project.build.sourceEncoding>UTF-8</project.build.sourceEncoding>
    <project.reporting.outputEncoding>UTF-8</project.reporting.outputEncoding>
    <mysql.connector.version>5.1.38</mysql.connector.version>
    <jtds.connector.version>1.3.1</jtds.connector.version>
</properties>

<dependencies>
    <dependency>
        <groupId>javax</groupId>
        <artifactId>javaee-api</artifactId>
        <version>7.0</version>
    </dependency>

    <dependency>
        <groupId>org.apache.httpcomponents</groupId>
        <artifactId>httpclient</artifactId>
        <version>4.5.2</version>
    </dependency>

    <dependency>
        <groupId>org.apache.commons</groupId>
        <artifactId>commons-lang3</artifactId>
        <version>3.4</version>
    </dependency>

    <dependency>
        <groupId>commons-httpclient</groupId>
        <artifactId>commons-httpclient</artifactId>
        <version>3.1</version>
    </dependency>

    <!-- JDBC -->
    <dependency>
        <groupId>mysql</groupId>
        <artifactId>mysql-connector-java</artifactId>
        <version>${mysql.connector.version}</version>
    </dependency>
    <dependency>
        <groupId>net.sourceforge.jtds</groupId>
        <artifactId>jtds</artifactId>
        <version>${jtds.connector.version}</version>
    </dependency>

    <!-- REST -->
    <dependency>
        <groupId>javax.ws.rs</groupId>
        <artifactId>javax.ws.rs-api</artifactId>
        <version>2.0.1</version>
    </dependency>

    <dependency>
        <groupId>com.thoughtworks.xstream</groupId>
        <artifactId>xstream</artifactId>
        <version>1.4.9</version>
    </dependency>

    <dependency>
        <groupId>commons-beanutils</groupId>
        <artifactId>commons-beanutils</artifactId>
        <version>1.9.2</version>
    </dependency>

    <dependency>
        <groupId>net.sf.json-lib</groupId>
        <artifactId>json-lib</artifactId>
        <version>2.4</version>
        <classifier>jdk15</classifier>
    </dependency>

    <!--  Email -->
    <dependency>
        <groupId>javax.mail</groupId>
        <artifactId>javax.mail-api</artifactId>
        <version>1.5.5</version>
    </dependency>

    <dependency>
        <groupId>javax.activation</groupId>
        <artifactId>activation</artifactId>
        <version>1.1.1</version>
    </dependency>

    <!-- POI -->
    <dependency>
        <groupId>org.apache.poi</groupId>
        <artifactId>poi</artifactId>
        <version>3.14</version>
    </dependency>

    <dependency>
        <groupId>org.apache.poi</groupId>
        <artifactId>poi-ooxml</artifactId>
        <version>3.14</version>
    </dependency>       
</dependencies>

<build>
    <plugins>
        <plugin> 
            <groupId>org.apache.maven.plugins</groupId>
            <artifactId>maven-dependency-plugin</artifactId>
            <version>2.8</version>
            <executions>
                <execution>
                    <id>copy-dependencies</id>
                    <phase>prepare-package</phase>
                    <goals>
                        <goal>copy-dependencies</goal>
                    </goals>
                    <configuration>
                        <outputDirectory>${project.build.directory}/classes/lib</outputDirectory>
                        <includeScope>runtime</includeScope>
                    </configuration>
                </execution>
            </executions>
        </plugin>
        <plugin>
            <groupId>org.apache.maven.plugins</groupId>
            <artifactId>maven-jar-plugin</artifactId>
            <configuration>
                <archive>
                    <manifest>
                        <addClasspath>true</addClasspath>
                        <classpathPrefix>lib/</classpathPrefix>
                        <classpathLayoutType>repository</classpathLayoutType>
                        <mainClass>br.com.mentium.hrm.agent.Agent</mainClass>
                    </manifest>
                </archive>
            </configuration>
        </plugin>
    </plugins>
</build>

And when I issue a mvn clean install, maven works its magic and I get a 18MiB jar file. All good!

But when I try to run it with a java -jar agent.jar, I get the following exception:

Exception in thread "main" java.lang.NoClassDefFoundError: org/apache/http/client/methods/HttpUriRequest
at br.com.mentium.hrm.agent.service.Runner.<init>(Runner.java:22)
at br.com.mentium.hrm.agent.Agent.execute(Agent.java:86)
at br.com.mentium.hrm.agent.Agent.main(Agent.java:58)
Caused by: java.lang.ClassNotFoundException: org.apache.http.client.methods.HttpUriRequest
at java.net.URLClassLoader.findClass(URLClassLoader.java:381)
at java.lang.ClassLoader.loadClass(ClassLoader.java:424)
at sun.misc.Launcher$AppClassLoader.loadClass(Launcher.java:331)
at java.lang.ClassLoader.loadClass(ClassLoader.java:357)
... 3 more

The lib that contains this file (it's an interface) has been added to the jar's lib folder, as highlighted bellow.

As per the notation on the pom.xml file above, all libs are included on the jar and stated in the MANIFEST.MF file.

What am I missing here?